LIGO diary 2002--report #0

by Wm. Robert Johnston
last updated 6 August 2002

Saturday, 6 July 2002: (I'm not there yet)

I'm off to LIGO in Louisiana for a month tomorrow. I am spending four weeks at the LIGO gravitational wave observatory near Livingston, Louisiana (like I did last summer). Besides me there are two other students from the Univ. of Texas-Brownsville physics dept. on this trip. This is connected to my studies for a masters degree in physics (towards which I have completed one year).

My little reports from last summer I put together with pictures on my web site; you can see them or a discussion of the physics of this here. But here's a brief explanation:

LIGO is trying to detect gravitational waves, which are predicted by Einstein's theory of general relativity. This theory explains gravity not as a force but as a warping or curving of space and time (space-time).

Stick your finger in some water and wiggle it. Ripples radiate from your finger, depending on how you wiggle it. In a sense, this is because the water can't adjust fast enough to the motion of your finger.

If a very massive, very dense object "wiggles" fast enough, it will produce ripples in space-time like your finger did in the water. These ripples of gravity are gravitational waves (GWs). They can be caused by such things as colliding black holes or neutron stars.

GWs cause stretching and compressing of space. If a GW was passing through your computer monitor coming toward you (or away from you), it would alternately stretch and compress in (say) an up and down direction. At the same time the opposite would be happening in the sideways direction (up-and-down compression and sideways stretching, then vice versa, then back again, over and over).

This stretching and compressing is tiny-the change in length and width for the whole Earth is still much smaller than an atom. This is where LIGO comes in. LIGO is Laser Interferometer Gravitational-wave Observatory:

Take two straight metal tubes, each (4 km) 2-1/2 miles long. Join them to form an "L". At the corner put a laser which can be split into two beams, one going down each tube, or arm of the "L". Put mirrors at each end to reflect the beam back to the corner. By recombining the beams at the corner, you can use the fact that they interfere with each other to compare the lengths of the arms.

This is LIGO oversimplified. If a GW goes by, the lengths of the arms will change in comparison to each other. In practice, the change in length for LIGO's 4 km (2-1/2 mile) arms is a small fraction of the diameter of the nucleus of an atom. So there are some technical challenges.

LIGO actually has two of these facilities, one in Washington state and the one in Louisiana. By comparing the results, they can better eliminate false alarms from things like cars crossing the cattle guard at the Louisiana site.

About now they were planning to start the first science run, when the two sites would gather scientific data at the same time. However, a problem in Washington has delayed that at least a month.

They may have me working with some other students on identifying some of the seismic noise that affects readings. (Last summer one of our students was in the group tracking down vibrations that turned out to be from logging operations in the area.) When I'm not doing that, I'll be working with data analysis software for the stochastic background group. This group (which includes faculty from UTB) is hoping to identify a cosmic background "noise" of GWs. This is also likely to be my master's thesis topic.